1. Field of the Invention
The present invention relates to a zoom lens, and an image pickup device provided with the same, and is well adaptable to an image pickup device using a solid-state image pickup element, such as a video camera, an electronic still camera, a broadcast camera, or a monitoring camera, or to an image pickup device such as a camera using a silver halide film.
2. Description of the Related Art
A photographing optical system in an image pickup device has been demanded which uses a high-performance zoom lens having a high zoom ratio and which enables a reduction in the size of the system. An image pickup device has also been demanded which has an autofocus (automatic focus detection) mechanism and which is capable of photographing not only a still image but also a moving image. When operating noise from the autofocus mechanism is load during the photographing of a moving image, an image pickup device having the autofocus mechanism might record the operating noise with the moving image. Therefore, an image pickup device having an autofocus mechanism has been demanded which is capable of performing quiet and quick focusing.
U.S. Pat. No. 8,331,035 describes a 4-unit zoom lens including four lens units having respectively a positive refractive power, a negative refractive power, a positive refractive power, and a negative refractive power in the order from an object side to an image side, wherein focusing is carried out by using the fourth lens unit to satisfactorily photograph a still image and a moving image. U.S. Pat. No. 8,451,549 and Japanese Patent Application Laid-Open No. 2000-180722 describe a 5-unit zoom lens including five lens units having respectively a positive refractive power, a negative refractive power, a positive refractive power, a negative refractive power, and a positive refractive power, in the order from, an object side to an image side, wherein focusing is carried out by using the fourth lens unit to satisfactorily photograph a still image and a moving image.
A zoom lens used in an image pickup device is required to be compact (downsized) as a whole, to have a high zoom ratio (high magnification ratio), and to have high optical performance within the entire zoom range. In most zoom lenses, a focus lens unit is moved in synchronization with zooming in order to prevent a focus variation during the zooming. When the focus lens unit and other lens units are controlled by different driving units, it becomes difficult to move the focus lens unit completely in synchronization with the zooming due to an electrical control delay or speed limit of motors, for example, whereby defocusing occurs upon the zooming.
In this case, the defocusing upon the zooming can be reduced by quickly operating the focus lens unit. However, this operation increases the operating noise. Therefore, use of a focusing system and zooming system capable of easily preventing the defocusing upon the zooming is required. When a lens unit moving with the zooming is used as a focus lens unit, the focus lens unit has to be provided with two types of moving mechanisms, which are a moving mechanism for zooming and a moving mechanism for focusing. It is also necessary to move the focus lens unit very fast.
When the focus lens unit is provided with two types of moving mechanisms, the whole system becomes large in size, and operating noise during focusing increases. Accordingly, a zoom lens used in an image pickup device has to have a focusing mechanism that can easily perform quiet and quick focusing and that can easily prevent defocusing during zooming. In particular, since the operating noise during focusing is mainly determined by the weight of a focus lens unit, it is important to appropriately set an arrangement, size, and weight of the focus lens unit in an optical path.